I just filled up the Abarth for the first time. I cannot give an accurate assessment of it's efficiency because it was not full when I bought it. The instant and average readouts generally point to around 30ish miles per gallon since I bought it.
Fuel efficiency is only one part of it though. Being less than half the weight and certainly less than half the size of my Rover, but still doing the same job of hauling my butt and a few things back and forth to work, it's physical size means a smaller footprint on the world as I go down the road. I use less fuel and oxygen, produce less CO2 and CO, put less wear and tear on the streets, and my expendables are much smaller as well as the car uses smaller tyres and suspension components to get the same job done.
The fact that it is much more fun on the street is also a plus, just as the rover is more fun off of it. In their respective environments, each excels.